Most car manufacturers use a urea-injection system, often called AdBlue, which uses a chemical catalyst to make sure unburnt fuel doesn’t get into the exhaust.

But VW says it can meet the regulation without the AdBlue system on many of its cars.

In 2013, the International Council on Clean Transportation teams up with West Virginia University for a study on the Volkswagen diesel cars.

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“We had no cause for suspicion,” John German, from the ICCT, said in an interview with Bloomberg. “We thought the vehicles would be clean.”

The study tests three cars in real-world conditions — a 2012 VW Jetta, a 2013 VW Passat and a BMW X5 SUV, under both laboratory and road conditions, finding huge differences in the amount of harmful emissions.

The group tests one on nearly 4,000 kilometres of highway driving between California and Washington State.

The tests find that the Volkswagen Jetta exceeds nitrous oxide caps by 15 to 35 times, with the Passat exceeding emissions caps by 5 to 20 times.

Meanwhile the BMW met all the standards under normal driving conditions.

The two groups alert the California Air Resources Board and Environmental Protection Agency in 2014.

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The EPA and CARB put the findings to Volkswagen.

The company disputes the test results “citing various technical issues” but implement a voluntary recall of nearly 500,000 cars in December 2014 to put in a software patch they claim will fix the issue.

It doesn’t and CARB and the EPA keep pushing to find out why the cars’ own diagnostics systems don’t register the high emissions under test conditions.

Volkswagen says the study is flawed, blaming “various technical issues” for the results.

Volkswagen dispute the test results but implement a voluntary recall of nearly 500,000 TDI cars in December 2014 to put in a software patch they claim will fix the issue.

It doesn’t and CARB and the EPA keep pushing to find out why the cars’ own diagnostics systems don’t register the high emissions under test conditions.

The tests find the root cause of how Volkswagen got its cars to pass the tests – discovering the software called “the switch.”

If it sensed that the car was being driven on a road rather than in a lab, it switched to a separate callibration which turned off the exhaust controls.

People have speculated this was done to keep the cost of the cars down, tricking the testers into thinking they were fine without the AdBlue systems used by other manufacturers.

Volkswagen finally admits the scheme on September 3 to the EPA and CARB.

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The company was confronted by the authorities with evidence of the software scam. Running out of excuses, the company admits to gaming the tests.

On September 18, the EPA go public with the findings of their Volkswagen tests.

“Using a defeat device in cars to evade clean air standards is illegal and a threat to public health,” says assistant administrator for the Office of Enforcement and Compliance Assurance Cynthia Giles.

The fine could be as high as $37,500 per vehicle for the violations, a total of more than $18 billion ((£11.8 billion). The U.S. Justice Department and German authorities also begin investigations.

When markets open on the following Monday, Volkswagen stock plunges more than 20%

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Volkswagen experiences its biggest one-day drop in six years as the potentially huge fine spooks investors.

CEO Martin Winterkorn releases an apology, saying “I personally am deeply sorry that we have broken the trust of our customers and the public.”

On September 22, Volkswagen admits the emissions scam is far more widespread, saying it could affect 11 million cars.

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Once again, the shares go into meltdown and another 20% is wiped off the value of the company.

Volkswagen issued a profit warning setting aside €6.5 billion (£4.7 billion) to “cover the necessary service measures and other efforts to win back the trust of our customers.” They add: “discrepancies relate to vehicles with Type EA 189 engines, involving some eleven million vehicles worldwide.”

Volkswagen CEO Martin Winterkorn steps down after appearing in a video saying he’s staying at the company

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Volkswagen AG Chief Executive Officer Martin Winterkorn, who during nearly a decade at the helm catapulted VW to the top spot in global sales, stepped down on Wednesday.

“Volkswagen needs a fresh start — also in terms of personnel,” Winterkorn said in a statement. “I am clearing the way for this fresh start with my resignation.”